Determining Depth to Groundwater and Ground Water Elevation in a Monitoring Well

Written Date:

March 1, 2003

Approved Date:

Purpose:

To measure water elevation in monitoring wells in a consistent manner. NDEQ uses this information primarily to determine ground water flow direction, and therefore, it is in the best interest of everyone to have accurate well measurements and land surface elevations.

Equipment/Materials Needed:

Measuring Tape: this can be a steel tape or an electric tape. Measuring tapes that may stretch or easily break (i.e. fiberglass) should not be used.

Appropriate weight for end of tape.

Clean rag (for steel tape method).

Blue carpenter’s chalk (for steel tape method).

Water level measurement table to record measurements.

Procedures:

1. One-time procedures, to be done prior to first water level measurement.

1.1. Determine where the Measuring Point (MP) is for each well. The MP should be clearly marked and described on the water level measurement table so a person who has not previously measured the well will know where to measure.

1.1.1. A common MP is the highest point of the casing (see diagram). It should be a notch, scratch, or other permanent visible mark. Describe the MP on your water level measurements table.

1.2. Determine the MP relative or absolute elevation for all wells. In order to tell the ground water flow direction from onsite well water measurements, one must know the relative or absolute MP elevation.

1.2.1. MP relative elevation can be determined by surveying all of the wells relative to a common point. The common point should be assigned a relative elevation (i.e. 300.00). The common point can be some part of permanent structure, such as a building corner or concrete driveway. Record the location of the common point on the site plan.

1.2.1.1. Hire a professional land surveying company to perform this.1.2.1.2. An accuracy of at least 0.01 foot is necessary.

1.2.2. Absolute elevation is the feet above mean sea level. Topographic maps from the United States Geological Survey have elevation in feet above mean sea level marked on them, usually on maps contoured to the nearest 10 or 20 feet. Because this contour interval is usually too broad to be able to tie a particular well to an exact elevation, the wells’ locations and MP elevations must be surveyed and compared to a known benchmark or other known elevation. Note: In Nebraska, absolute elevations will range from approximately 800.00 to 5,300.00 feet above mean sea level.1.2.3. Record the MP elevation for each well on your water level measurement table (see line E on table).

1.3. Determine land surface elevation at all wells.

1.3.1. Measure the distance from the land surface to the MP and record on your water level measurement table (see line F on table). Always use the same MP, each time you measure the water levels.1.3.2. The land surface elevation = (MP elevation – Distance between MP and land surface)(see table).

1.4. A water level measurement table may be used for several water level measuring events. Make copies of the table with the preliminary information filled in, to be used in the future.

2. Steel Tape

2.1. Open all of the wells to be measured and allow the water level to stabilize prior to collecting the water level measurement.2.2. Calibration: none. Steel tapes are commonly marked in increments of 0.01 foot.2.3. Locate the Measuring Point (MP). 2.4. Prepare tape. The steel tape must have a weight attached to the end. If the steel tape is new, the black sheen must be dulled in order for the chalk to stick (steel wool or fine sandpaper). Wipe the tape clean with a clean rag. Chalk the tape to a distance above where you expect the water level to be. For example, if the last time the well water level was measured the depth to water was 11 feet, chalk the last five feet of the tape.2.5. Lower the weighted tape into the well until the lower end of the tape is submerged below the water. Do this slowly to prevent splashing. Continue to lower the end of the tape into the well until the next graduation (a whole foot mark) is at the MP. Record this number in the MP HOLD column (see table, Col. A).2.6. Rapidly (and carefully) bring the tape to the surface before the wetted chalk mark has time to dry (and becomes difficult to read). Record the wetted chalk mark measurement in the WETTED CHALK MARK column (see table, Col. B).2.7. Subtract the “Wetted Chalk Mark” number from the “MP Hold” number and record this value in the DEPTH TO WATER FROM MP column (see table, Col C).2.8. Subtract the “Depth to Water” from MP from the “Relative or Absolute elevation of the MP”. Record this in the WATER ELEVATION column (see table, Col. D).2.9. Record other important information on table, such as time, date, weather, and person doing the work.2.10. Replace cap and secure the well when the work is completed.

3. Electric Tape.

3.1. Open all of the wells to be measured and allow the water level to stabilize prior to collecting the water level measurement.3.2. Calibration: Check with the manufacturer’s instructions for your particular instrument model.

3.2.1. Some electric tapes have steel bands at 5-foot (or other) intervals. Check to make sure they are still correctly spaced.

3.3. Check the circuitry and batteries of the electric tape by dipping the probe into water and checking its operation prior to putting it in the well.3.4. Lower the electrode probe slowly into the well to prevent splashing. When the electric tape beeps or the light goes on, you have reached the water level surface. Record this value from the tape on the table (see table, Col. C) in the DEPTH TO WATER FROM MP column.3.5. Subtract the “Depth to Water from MP” from the “Relative or Absolute elevation of the MP”. Record this in the WATER ELEVATION column (see table, Col. D).3.6. Record other important information on table, such as time, date, weather, and person doing the work.Replace cap and secure the well when the work is completed.

Water Level Measurements Tableis available in the attached Adobe Acrobat file